Penning vacuum meter

ABSTRACT

The invention relates to a Penning vacuum meter ( 1 ), comprising a cathode and an anode. According to the invention, the cathode ( 11 ) consists at least mainly of titanium in order to avoid the detrimental effects of the cathode disintegrating.

BACKGROUND OF THE INVENTION

The present invention relates to a Penning vacuum meter comprising acathode and an anode.

A Penning vacuum meter is a cold cathode vacuum meter, the operation ofwhich is based on a cold discharge. It comprises a tube with twonon-heated electrodes, cathode and anode, between which a discharge isignited and maintained by means of a DC voltage (about 2 kV duringoperation), whereby said discharge is maintained in a steady state alsoat very low pressures. This is attained by making the path for theelectrons so long with the aid of a magnetic field, that their collisionrate with the gas molecules is sufficiently high in order to form therequired quantity of charge carriers to maintain the discharge.

In the tubes of Penning vacuum meters, the cathode material isdisintegrated by the ions which are accelerated in the direction of thecathode, particularly so in connection with coating processes involvingheavy noble gases (argon, xenon) being performed in the pressure rangebetween 10⁻⁴ and 10⁻² mbar. The disintegrated cathode material forms onthe walls of the measurement tube a conductive layer.

Generally, stainless steel is employed as the material for the cathode.Owing to the cathode disintegration process described (“sputtering”) theservice life of the cathode is limited. Moreover, disintegratedstainless steel cathode material is magnetic. Parts coming loose fromthe formed layers are capable of aligning themselves in the magneticfield thereby causing short circuits. In addition, parts of themeasuring cell, which actually should be electrically isolating (ceramicfeedthroughs) may become electrically conducting. These disadvantagesresult in an unstable readout or extinguishing of the discharge and thusin a failure of the measurement tube. Even so, the Penning vacuum meteris employed very often, since it is cost-effective and insensitive toair inrushes and vibrations.

SUMMARY OF THE INVENTION

It is the task of the present invention to substantially remove thedetrimental effects which occur owing to the disintegration effectdescribed.

This task is solved by employing titanium as the material for thecathode. Owing to the lower sputtering yield, sputtering of the cathodematerial is only slight when using titanium. Thus the service life ofthe cathode is improved and coating with conductive metal is reduced.The sputter products are non-magnetic so that they may no longer causethe described short circuits in the magnetic field. Therefore, Penningvacuum meters with titanium cathodes will provide a stable readout overa long time even in coating processes. Surprisingly, the ignitioncharacteristic is also improved.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and details of the present invention shall beexplained on the basis of the design example presented in the drawingfigures.

FIG. 1 is a side elevation in section of a Penning vacuum meterembodying the teachings of the present invention, and

FIG. 2 is a view of the V-shaped cathode panel shown in FIG. 1 takenalong lines 2—2. Cathode 11 consists mostly of titanium.

DESCRIPTION OF THE INVENTION

The depicted Penning vacuum meter 1 comprises housing 2 with flange 3for connecting the housing to a vacuum chamber or the like (not shown),and the magnet 4 arranged outside of the housing. The currentfeedthrough 6 extending from the connecting socket 5 into the inside ofthe housing 2 carries a ring or annular shaped anode 7 with ignition pin8.

The cathode 11 is an U-shaped panel, the arms 12, 13 of which arerectangular shaped and embrace the anode 7. The arms extendsubstantially in parallel to the plane of the anode ring 7. Cathode 11consists mostly of titanium.

The cathode panel 11 is fitted to the housing 2 using a tight fit. Thebase plate of the cathode 14 which joins the arms 12, 13 is located inthe area of the opening of the flange 3. The base plate is equipped withcut-outs 15 in order to ensure a sufficiently high conductance for thein process gases.

Depicted in drawing FIG. 2 is a front view of the anode looking at theinside of arm 12 of the U-shaped cathode panel 11. As shown in FIG. 2,the outside diameter of the anode is about equal to the width of eacharm and less than the length of each arm. The anode lies in a plane thatis coaxial with the central axis 20 of the cathode housing. The dashedlines 16 indicate that the inner surface area of arms 12 and 13 facingthe opening of the anode ring 7 have been roughened by sand blasting,for example. The ignition characteristic is improved by these roughenedsurfaces, since the points created promote field emissions by theelectrons.

A d.c. power supply 21 is provided to apply about 2 kV between thecathode and the anode when the meter is operating.

What is claimed is:
 1. A penning vacuum meter that includes: a cathodehousing having a flange at one end for connecting the housing to avacuum chamber and an opening passing axially through said housing; aU-shaped cathode, mounted in said housing, said cathode having twoopposed rectangular shaped arms connected by a flat base plate, saidarms being parallel to the axis of said housing and being tightly fittedagainst opposed inner walls of said opening with the base plate of thecathode passing across the opening at the flange; an annular shapedanode suspended within said opening between the arms of said cathode,said anode lying in a plane that is parallel with said arms, thediameter of the anode being substantially equal to the width of saidarms; and said cathode being formed of a material that will resistdisintegration when bombarded by accelerated ions when operating withina pressure range of between 10⁻⁴ and 10⁻² mbars.
 2. The Penning vacuummeter of claim 1 wherein the opposing surfaces of the cathode arms areroughened to produce points for promoting the field emission ofelectrons.
 3. The Penning vacuum meter of claim 1 wherein the base ofthe cathode contains a plurality of cutouts passing therethrough toensure high conductance for process gases.
 4. The Penning vacuum meterof claim 1 that includes a d.c. power supply for applying about 2 kVbetween the cathode and the anode.
 5. The Penning vacuum meter of claim1 that further includes a magnet surrounding the housing about saidcathode for extending the electron path within said housing.
 6. ThePenning vacuum meter of claim 1 wherein said cathode is fabricated of anon-magnetic material.